I'd love to see Affinity Publisher support tagged and accessible PDFs from the ground up, ideally with full support for PDF/UA. Many institutions and government bodies require this for public documents, and making it part of the program's logic could be a major selling point.
I am not convinced that the way in which InDesign implements this in is the best way of going about it; it tends to create a lot of extra work that most people don't bother with. Instead, it should be possible to use this structural information to make document creation more efficient and improve a print/Web workflow.